1. Field of the Invention
The invention relates to the adsorptive purification of the gas stream of vaporous or gaseous impurities in a sorption filter, where at least two layers of sorption material are present in the sorption filter.
2. Description of the Prior Art
Activated carbon is frequently used for the separation of vaporous or gaseous impurities from a gas stream, for instance, exhaust air. In order to recover the separated substances and to make the activated carbon adsorptive again, a regeneration by means of a heated gas is performed. As is known from German patent application No. 29 52 127 and from German patent application No. 29 36 873, a gaseous desorption medium is heated for the regeneration, the heated medium is driven through the sorption material layers, and the desorption medium loaded with desorbate is cooled after it has flowed through the sorption material layers to condense desorbate, at least in part, and the liquified desorbate component discharged. In order to avoid losses of desorption medium and recoverable desorbate, the desorption medium with the uncondensed desorbate residues is frequently returned to the heating location, is heated there and pushed again through the sorption material layers. Since the partial pressure of the desorbate can be lowered only to the equilibrium with the vapor saturation concentration corresponding to the cooling temperature reached, the desorption cannot be complete because of the partial residual pressure of the desorbate; therefore the sorption material retains its residual loading. Particularly high residual loadings remain, according to experience, if relatively thin layers of activated carbon (0.5 m and less) are employed. Thicker layers however lead to greater pressure losses and thereby to increased energy consumption.
It has now been found that this residual loading can still be too high if the desorption temperatures are relatively low and the condensation temperatures relatively high. This is often the case for economy and operating reasons because, as a rule, only steam with temperatures slightly above 100.degree. C. is available for the heating for desorption, and for the cooling for condensation, only cooling water of 10.degree. to 20.degree. C. is available. This low temperature difference between the desorption and the condensation temperature of 80.degree. to 90.degree. K. leads to reloading of the desorbed sorption material because of the high residual partial pressure, especially in the case of such driven-out admixtures (desorbate) with a low boiling point. The residual partial pressure is predetermined here by the condensation temperature and the absorptivity of the sorption material is influenced by the desorption temperature. The reloading of the desorbed sorption material results in a shortened service life (time between two desorptions) and leads in particular to a high concentration of the substances to be separated in the purified gas, at the beginning of the sorption phase.
In order to lower the vapor pressure of the desorbate in the desorption medium during the desorption, U.S. Pat. No. 3,534,529 proposes to provide a secondary adsorption filter instead of the condensation in the desorption loop. As a result, the partial pressure of the desorbate in the desorption medium practically disappears at least in the starting phase of the desorption. In order to maintain the low partial pressure over the entire desorption cycle, a design of the secondary adsorber is required which is uneconomical as measured by the design of the adsorption filters. The secondary adsorber must be capable of taking up the entire mass of desorbate stored in the adsorption filter without a breakthrough, especially since the temperature of the desorption medium increases toward the end of the desorption and because more cooling is required in order to obtain the high loading of the secondary adsorber. In spite of the high loadability of the sorption material of the secondary adsorber due to the high partial pressure prevailing during the desorption, this secondary adsorber cannot be designed substantially smaller than the sorption filter itself.